1. Field of the Invention
The present invention described in the specification relates to a detection technology of input operation or position input to a capacitive sensor device. Particularly, the invention proposed in the specification relates to a capacitance change measuring circuit of a capacitive sensor device, a capacitive sensor module, a method of measuring capacitance change of a capacitive sensor device, and an electronic device.
2. Description of the Related Art
In recent years, various types of position input devices exist. Examples thereof include a sensor device using a finger or an exclusive pointing device having electric characteristics equal to those of a finger. For such a sensor device, various types exist as usage. Discussion will be hereinafter made on a capacitive sensor module which includes a capacitive sensor device and a drive circuit thereof (capacitance change measuring circuit).
FIG. 1 and FIG. 2 illustrate an example of schematic structure of capacitive sensor modules. The capacitive sensor module includes a capacitive sensor device 1 and a circuit detecting the capacitance change thereof (hereinafter referred to as “capacitance change measuring circuit”) 11. The sensor device 1 has a structure in which a plurality of electrode patterns 5 are wired on one face of a tabular base material 3, and a plurality of electrode patterns 7 are wired on the other face of the tabular base material 3.
For example, the electrode patterns 5 on the top face side are wired to extend in the Y-axis direction, and the electrode patterns 7 on the bottom face side are wired to extend in the X-axis direction. The surface of the electrode patterns 5 on the top face side are covered with a protective film (not illustrated). Further, at each intersection of each electrode pattern 5 and each electrode pattern 7, a minute capacitance is formed, through which the electrode patterns 5 on the top face side are electrically connected to the electrode patterns 7 on the bottom face side.
In the case of the capacitive sensor device 1 that is used as a touch panel arranged on the surface of a display device, the base material 3, the electrode patterns 5, and the electrode patterns 7 are respectively made of a material having high transmissive characteristics in order to view the display screen. For example, for the base material 3, a glass substrate or a plastic film is used. Further, for example, for the electrode patterns 5 and the electrode patterns 7, an ITO electrode is used.
The capacitance change measuring circuit 11 is structured so that a closed circuit between the capacitance change measuring circuit 11 and the capacitive sensor device 1 is formed, and change of electric characteristics generated in the closed circuit is detected, through which presence of operation and an operation position are able to be detected. The closed circuit includes an extraction wiring pattern, the electrode patterns 5, the capacitance formed in the intersection of the electrode patterns, the electrode patterns 7, an extraction wiring pattern, and the capacitance change measuring circuit 11. For details of an equivalent circuit, a description will be given later.
FIG. 3 illustrates an example of existing circuit of capacitive sensor modules. The example of existing circuit corresponds to the invention disclosed in Published Japanese Translation No. 2002-530680 (JP2002-530680A) of the PCT International Publication. The structure of the capacitive sensor device 1 illustrated in FIG. 3 illustrates capacitance components of the closed circuit as a measurement target in each processing timing. Ctp is a capacitance component statically formed in each intersection of each electrode pattern 5 and each electrode pattern 7.
Cbulk is a static capacitance component obtained by the total of a capacitance component of the electrode patterns 5 and 7, a capacitance component of the extraction wiring connecting the electrode patterns 5 and 7 to the capacitance change measuring circuit 11, a capacitance component of the base material 3, and a capacitance component of an IC pin. Cf is a dynamic capacitance component generated between a finger or a device having electric characteristics equal to those of a finger and the electrode patterns 5/7. In FIG. 3, the all capacitance components existing on the measurement path (closed circuit) are expressed by Cx.
Further, the capacitance change measuring circuit 11 illustrated in FIG. 3 includes a voltage source Vr, a capacitance Cref, three analog switches S1, S2, and S3, a comparator 13, and a counter 15. In the circuit structure, the capacitance change measuring circuit 11 sequentially executes the following operations, and determines presence of input operation.    1. First, the all analog switches S1, S2, and S3 are controlled to be on-state. By this operation, electric charges of the capacitances Cref and Cx are discharged.    2. After the electric charges of the capacitances Cref and Cx are all discharged, the all analog switches S1, S2, and S3 are controlled to be off-state.    3. After that, only the analog switch S1 is controlled to be on-state. At this time, according to Kirchhoff low, a charge Q with a size equal to that of Cref and Cx is charged.    4. Next, the all analog switches S1, S2, and S3 are controlled to be off-state. After that, only the analog switch S2 is controlled to be on-state. Thereby, only the capacitance Cx on the measurement path is discharged.    5. The foregoing operations 2 to 4 are repeatedly executed. In result, as illustrated in FIG. 4, the voltage of the capacitance Cref is gradually increased. When a voltage in point A of FIG. 3 exceeds reference potential Vref, determination output of the comparator 13 is changed. Further, through the change of determination output, the repetition operations are controlled to be stopped.    6. As described above, until the determination output of the comparator 13 is changed, a count value of the counter 15 is outputted. The count value is a measurement value of the capacitance Cx on the measurement path. The size of the capacitance Cx on the measurement path in the case where a finger exists on the measurement path is different from the size of the capacitance Cx on the measurement path in the case where a finger does not exist on the measurement path. That is, the former size is different from the latter size by the portion of Cf. Therefore, by comparing the measured count value to the count value in the case where a finger does not exist, presence of a finger is able to be determined.